1. Field of the Invention
The present invention relates to a method and an apparatus for forming an image, applicable to printers and copiers.
2. Description of the Related Art
A typical image forming method employed by printers and copiers is a toner developing method (an electrostatic latent image method). This method attaches toner to an electrostatic latent image by electrostatic attractive force to develop the latent image. This method is mainly achieved as a powder developing technique (a dry toner developing technique) and as a liquid developing technique (a liquid toner developing technique).
The powder developing technique attaches charged coloring powder toner made of fine resin grains of 6 to 15 .mu.m in diameter to an electrostatic latent image with electrostatic attractive force.
The liquid developing technique disperses coloring powder toner made of resin in an insulating liquid serving as a dispersion liquid, electrically charges the toner in the liquid, and attaches the solution to an electrostatic latent image with an electrostatic attractive force. This technique employs very fine toner having a grain diameter of 0.1 to several microns to realize higher definition recording than the powder developing technique and provide excellent color reproducibility.
The liquid developing technique electrophoreses charged toner dispersed in a liquid with electrostatic attractive force so that the toner may attach to an electrostatic latent image. At this time, the toner moving toward the latent image receives the viscous resistance of the liquid, to slow down. Consequently, the toner takes a long time to attach itself to the latent image when providing the latent image with a proper density. Namely, the liquid developing technique involves a slow developing speed and a slow printing speed. Since the toner is made of very fine grains, it physically strongly adheres to an image carrier on which an electrostatic latent image is formed. As a result, after the image is transferred to a recording medium, the toner tends to remain on the image carrier, thereby deteriorating transferring efficiency. In view of this, new developing techniques that employ no electrostatic latent image have been developed based on the liquid developing technique. These new techniques change the affinity of an image carrier for ink or coloring liquid before attaching the ink to the image carrier. These techniques are simple forms of offset printing that attach oil-based ink to an image carrier by using a difference in the hydrophilic or lipophilic property of the image carrier. These techniques may collectively be called as an "ink developing technique" or an "affinity/repellency latent image technique." They are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 4-21844, 6-230616, and 6-127110.
The publication No. 4-21844 forms an image carrier from photochromic polymers so that the surface of the image carrier may reversibly change the chemical structure thereof in response to external signals applied to form a latent image on the surface. Namely, the area where the latent image is formed on the image carrier shows hydrophilicity and the other area thereof shows hydrophobicity. The latent image area is developed with water-soluble ink, and the ink on the image is transferred to a recording medium.
The publication No. 6-230616 employs a printing plate made of a conductive support layer (a first layer) having electric potential, an optical semiconductor layer (a second layer), and an electrical insulating layer (a third layer) having water repellency. When exposed to a laser beam, the second layer becomes conductive to inject charges from the first layer into the third layer. The charge-injected part changes from nonpolar to polar, i.e., from lipophilic to hydrophilic, to form a latent image that shows affinity for ink.
The publication No. 6-127110 disperses fine resin grains in a dispersion medium and heats and/or pressurizes the solution to attach the solution to a latent image formed on an image carrier that has liquid repellency. The part where the resin grains are adhering to the image carrier shows affinity for ink to visualize the latent image.
These techniques may have a sufficient developing speed. However, they have poor ink transferring efficiency because ink sticks to a latent image formed on the image carrier due to the improved affinity of ink to the-latent-image-formed part of the image carrier. Accordingly, these techniques are appropriate only when printing many copies of a single image.